1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device capable of electrical writing, reading, and erasing, and a manufacturing method thereof.
2. Description of the Related Art
In recent years, a market of a nonvolatile memory capable of electrically rewriting data and storing data even when power is turned off has been expanded. The nonvolatile memory has a similar structure to that of MOSFET (Metal Oxide Semiconductor Field. Effect Transistor) and has a feature in that a region capable of accumulating charge for a long time is provided over a channel formation region. This charge accumulating region is formed over an insulating layer and is insulated and isolated from a peripheral region; accordingly, the charge accumulating region is also referred to as a floating gate. A control gate is provided over the floating gate with another insulating layer interposed therebetween.
In a so-called floating gate type nonvolatile memory having such a structure, an operation in which charge is accumulated in a floating gate and the charge is discharged is performed by voltage applied to a control gate. That is, when the charge held in the floating gate is taken in and out, data is stored. Specifically, the charge is injected into or extracted from the floating gate by application of high voltage between a semiconductor layer in which a channel formation region is formed and the control gate. It is said that, at this time, fowler-nordheim (F-N) type tunnel current (NAND type) or a thermoelectron (NOR type) flows through an insulating layer formed over the channel formation region. Accordingly, the insulating layer is also referred to as a tunnel insulating layer.
The floating gate type nonvolatile memory is required to have a characteristic of being able to hold charge stored in the floating gate for more than ten years in order to assure reliability. Therefore, the tunnel insulating layer is required to be formed to be thick enough to make tunnel current flow and to have a high insulating property so that the charge is not leaked.
In addition, various structures of the nonvolatile memory are considered in order to achieve low writing voltage, improvement in a charge holding characteristic, or low cost of the nonvolatile memory. For example, a structure in which a memory transistor is provided over a glass substrate or a plastic substrate is given (for example, Patent Document 1: Japanese Published Patent Application No. 2006-13534).